1. Field of the Invention
The invention relates to a method of deaerating or degassing a condensate in a water/steam loop of a gas and steam-turbine plant. Furthermore, it relates to an arrangement for carrying out the method.
In a gas and steam-turbine plant (combined cycle gas turbine plant), the heat contained in the exhaust gas from the gas turbine is utilized for generating steam for the steam turbine. In the process, the steam expanded in the steam turbine is condensed in a water/steam loop, and the feedwater accumulating as a result is evaporated in a waste-heat boiler through which the hot exhaust gas from the gas turbine flows. In order to avoid corrosion in the water/steam loop, the gas released in the condensate, in particular oxygen, is normally removed by thermal deaeration or degassing. To that end, a deaerator or degasser is normally provided in a feedwater tank connected downstream of the condenser of the steam turbine, the condensate, which flows to the feedwater and is slightly undercooled at a temperature of 8 to 15 K, being heated up to boiling point in the feedwater tank by a heating medium.
2. Description of the Related Art
In a prior art method of heating and degassing the condensate collected in the feedwater tank disclosed in the publication "Handbuch der Energie" [Energy Handbook], vol. 7, 1984, pages 100-107, steam is used as heating medium, which for this purpose is additionally generated in a low-pressure evaporator in the waste-heat boiler. As shown in European Patent Application No. 0 515 911, steam in the form of tapped steam from a low-pressure part of the steam turbine may alternatively be used as heating medium. In both cases, however, the heating steam is lost from the steam driving the steam turbine and, accordingly, the efficiency of the plant is limited.
European Patent Application EP 0 037 845, in addition, discloses a combined cycle gas turbine plant in which a separate preheating coil, which is conducted via the waste-heat boiler, is provided to preheat the feed water. JP-A-55/109708 discloses a combined cycle gas turbine plant in which preheated feed water is introduced to heat the feed water tank. However, in those systems, considerable expenditure on equipment is involved to provide the systems required for condensate deaeration.